ISPConfig itself is not needing any resources, so your questions are basically which requirements does an apache webserver with postfix and amavisd has.
I recommend that you have at least about 2 GB ram, as ram is so ceap it does not hurt to have even more as more ram will speed up your server (for example as mysql and the kernel can use more ram for caching). The bandwidth depends on the page requests that you expect for the sites and not the number of sites. You might have 30 sites with 10 000 requests per site and month and you can have one site which has 10 million requeststs per month.
One site note from my site. For a hosting server I personally wont choose SuSE Linux. Better use Debian. You will see that its more stable and faster on the long term. And if you dont have much ram, use 32Bit Linux and not 64 Bit as 64Bit applications use more RAM then thir 32Bit counterparts. So if you dont have more then 3 GB Ram, I recommend to use a 32Bit Linux.